[RFC] Run GitHub Actions only on `optuna/optuna`.

GitHub Actions are running on forked repositories (for example, https://github.com/himkt/optuna/actions). If an example CI fails on a forked repo, a user could receive an alert mail like the following.

Run failed for master (d6eb3a8)

Repository: himkt/optuna
Workflow: examples
Duration: 9 minutes and 51.0 seconds
Finished: 2020-04-13 15:12:06 UTC

View results: https://github.com/himkt/optuna/actions/runs/77229227

Jobs:
  * build (3.5) cancelled (2 annotations)
  * build (3.6) failed (1 annotation)
  * build (3.7) cancelled (2 annotations)
  * build (3.8) cancelled (2 annotations)

I think it doesn’t need to run GitHub Actions example and stale on forked repositories. I’m wondering we can stop them using jobs.<job_id>.if.

ref. https://github.community/t5/GitHub-Actions/Stop-github-actions-running-on-a-fork/td-p/51499

[himkt]

Thank you for reactions, I’ll make a PR later.

@hvy Thank you for pointing out, you’re right! Let me note disabling email notification (Settings -> Notifications -> GitHub Actions and remove checks on Email) also works for a quick fix. (But this is the account-wide configuration) (I also think it would be better to avoid making collaborators update their settings manually.)

[sile]

I think it doesn’t need to run GitHub Actions example and stale on forked repositories.

I definitely agree with you.